This invention relates to a battery that operates reversibly at ambient temperature. More specifically, this invention relates to an improvement of the electrolyte and positive electrode.
The recent advances in microelectronics technology have been remarkable and as is typically seen in memory backup power supplies for various electronic apparatuses, it is common practice today to accommodate batteries within the apparatus and to make an integral assembly with electronic devices and circuit. Under the circumstances, a need has arisen to reduce the size, weight and thickness of batteries and to increase their energy density. In the field of primary batteries, small and lightweight lithium batteries have already been commercialized but their utility is quite limited. This may be explained as follows: liquid electrolytes, in particular those which have ionic compounds dissolved in organic electrolytes, have conventionally been used with batteries that utilize an electrochemical reaction or other electrochemical devices such as electric double layer capacitors and electrochromic devices but such liquid electrolytes suffer from problems such as low long-term reliability and splashing in the sealing step because there is high likelihood of electrolyte leakage from parts of the battery and dissolution or evaporation of electrode materials.
With a view to improving the liquidtightness and long-term storage stability of electrochemical devices, ion-conductive high-molecular weight compounds having high ion conductivity have been reported and are still being studied as a means of solving those problems. The ion-conductive high-molecular weight compounds that are under the current review include straight-chained, reticular crosslinked or comb-shaped homopolymers or copolymers comprising ethylene oxide as building blocks. With a specific purpose of increasing the ion conductivity at low temperatures, it has been proposed that crystallization be prevented by using reticular crosslinked or comb-shaped polymers and this idea has already been implemented. In particular, ion-conductive high-molecular weight compounds using such reticular crosslinked polymers are useful since they have high mechanical strength and good ion conductivity at low temperatures.
When one wants to use those ion-conductive high-molecular weight compounds as electrolyte in electrochemical devices, it is necessary to reduce the thickness of the electrolyte in order to lower the internal resistance. In the case of ion-conductive high-molecular weight compounds, thin films of uniform thickness can be easily worked into a desired shape but not all methods are completely satisfactory for use in practical applications. Examples of the methods that have so far been proposed include: i) casting a solution of an ion-conductive high-molecular weight compound and removing the solvent by evaporation; ii) coating a polymeric monomer or macromer onto a substrate and polymerizing it thermally; and iii) curing an ion-conductive high-molecular weight compound by exposure to active radiation. The second method (thermal polymerization) is convenient and has been used most commonly. However, the time required to complete thermal polymerization is very long, making it difficult to accelerate the production rate. Further, a temperature gradient tends to develop in the heating furnace and the need to heat in an inert gas atmosphere has unavoidably increased the size of the heating furnace and associated equipment.